Proppants for hydraulic fracturing technologies

ABSTRACT

The invention is directed to systems and methods for forming and using proppant particles having desirable attributes.

RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.61/253,350, filed on Oct. 20, 2009. The entire teachings of the aboveapplication are incorporated herein by reference.

FIELD OF APPLICATION

This application relates generally to systems and methods for fracturingtechnologies.

BACKGROUND

In the process of acquiring oil and/or gas from a well, it is oftennecessary to stimulate the flow of hydrocarbons via hydraulicfracturing. The term fracturing refers to the method of pumping a fluidinto a well until the pressure increases to a level which is sufficientto fracture the subterranean geological formations containing theentrapped materials. This results in cracks and breaks that disrupt theunderlying layer to allow the hydrocarbon product to be carried to thewell bore at a significantly higher rate. Unless the pressure ismaintained, the newly formed openings close. In order to open a path andmaintain it, a propping agent or proppant is injected along with thehydraulic fluid to create the support needed to preserve the opening. Asthe fissure is formed, the proppants are delivered in a slurry where,upon release of the hydraulic pressure, the proppants form a pack or aprop that serves to hold open the fractures.

The behavior of the proppants in the fracturing fluids has to meetcertain performance requirements. First, in order to place the proppantsinside the fracture, proppant particles are suspended in a fluid that isthen pumped to its subterranean destination. To prevent the particlesfrom settling, a high viscosity fluid is often required to suspend them.The viscosity of the fluid is typically managed by addition of syntheticor natural based polymers. If the particles were more buoyant, a lessviscous suspension fluid could be used, which would still convey theparticles to the target area but which would be easier to pump into theformation. Second, it is desirable that the proppants remain where theyare placed throughout the lifetime of the well after they have beeninjected into a fracture line. If changes within the reservoir duringwell production force the proppants out of position, productionequipment can be damaged, and the conductivity of the reservoirformation can be decreased as its pores are plugged by the displacedproppants. Third, the proppants in the system should be resistant toclosure stress once they are placed in the fracture. Closure stressescan range from 1700 psi in certain shale gas wells, up to and exceeding15,000 psi for deep, high temperature wells. Care must be taken that theproppants do not fail under this stress, lest they be crushed into fineparticles that can migrate to undesirable locations within the well,thereby affecting production. Desirably, a proppant should resistdiagenesis during fracture treatment. The high pressures andtemperatures combined with the chemicals used in fracturing (frac)fluids can adversely affect the proppant particles, resulting in theirdiagenesis, which can eventually produce fine particulate matter thatcan scale out and decrease the productivity of the well over time.

Current proppant systems endeavor to address these concerns, so that theproppants can be carried by the fracturing fluids, can remain in placeonce they arrive at their target destination, and can resist the closurestresses in the formation. One approach to preparing suitable proppantsincludes coating the proppant materials with resins. A resin-coatedproppant can be either fully-cured or partially-cured. The fully-curedresin can provide crush resistance to the proppant substrate by helpingto distribute stresses among the grain particles. A fully-cured resincan furthermore help reduce fine migration by encapsulating the proppantparticle. If initially partially-cured, the resin may become fully curedonce it is placed inside the fracture. This approach can yield the samebenefits as the use of a resin that is fully-cured initially. Resins,though, can decrease the conductivity and permeability of the fracture,even as the proppants are holding it open. Also, resins can fail, sothat their advantages are lost. Finally, resin-based systems tend to beexpensive.

Another approach to preparing suitable proppants involves mixingadditives with the proppant itself, such as fibers, elastomericparticles, and the like. The additives, though, can affect therheological properties of the transport slurry, making it more difficultto deliver the proppants to the desired locations within the fracture.In addition, the use of additives can interfere with uniform placementof the proppant mixture into the fracture site. While there are knownmethods in the art for addressing the limitations of proppant systems,certain problems remain. There is thus a need in the art for improvedproppant systems that allow precise placement, preserve fractureconductivity after placement, and protect well production efficiency andequipment life. It is further desirable that such improved systems becost-effective.

SUMMARY

The invention is directed to composite proppant particles, and systemsand method for the use thereof.

In certain aspects, the invention is directed to a composite proppantparticle, comprising:

a proppant particulate substrate,

an inner polymeric layer deposited on the particulate substrate, and

a hydrophilic outer coating layer deposited upon the first layer or onan optional intermediate layer,

wherein the inner layer comprises a first polymeric material and theouter layer comprises a second polymeric material.

In another aspect, the invention is directed to an aggregate proppantparticle comprising:

a first proppant particulate substrate, and

a second proppant particulate substrate affixed thereto,

wherein the first proppant particulate substrate comprises a densematerial and the second proppant particulate substrate comprises a lowerdensity material.

In one embodiment of the composite proppant particle of the invention,the proppant particulate substrate is an aggregate proppant particle.

The invention also encompasses a proppant system, wherein the systemcomprises the composite proppant particle and/or aggregate proppantparticle of the invention and a fluid delivery vehicle.

In another embodiment, the invention is directed to a treatment fluidcomprising a fracturing fluid and a multiplicity of composite proppantparticles, wherein the composite proppant particles are suspended in thefracturing fluid and wherein the composite proppant particle comprise:

a proppant particulate substrate,

an inner polymeric layer deposited on the particulate substrate, and

a hydrophilic, outer coating layer deposited upon the first layer or onan optional intermediate layer,

wherein the inner layer comprises a first polymeric material and theouter layer comprises a second polymeric material.

The invention also encompasses a method of fracturing a subterraneangeological formation comprising introducing into said formation atreatment fluid wherein the fluid comprises suspended composite proppantparticles, wherein each composite proppant particle comprises:

a proppant particulate substrate,

an inner polymeric layer deposited on the particulate substrate, and

a hydrophilic, outer coating layer deposited upon the first layer or onan optional intermediate layer,

wherein the inner layer comprises a first polymeric material and theouter layer comprises a second polymeric material.

DETAILED DESCRIPTION

Disclosed herein are compositions and systems comprising proppantparticles and methods for forming and using proppant particles havingdesirable attributes such as a lower friction coefficient in the wetstate, good bonding adhesion with each other after placement in afracture site, resistance to uncontrolled fines formation, andhydrophilic surface properties to prevent fouling. In embodiments, thedisclosed systems for forming proppant particles can be applied to thetypes of proppant substrates most widely used, e.g., sand and ceramics.In other embodiments, the proppant particles can be formed from avariety of substrates, as would be available to those having ordinaryskill in the art. In certain embodiments, the proppant particles can befabricated so that they resist crush or deformation, so that they resistdisplacement, and so that they can be suspended in less viscous fluidcarriers for transporting into the formation.

In embodiments, the surface of a proppant particulate substrate can becoated with a selected polymer, either as a single layer or as a seriesof multiple coating layers. The coating (either single layer ormultilayer) can show switchable behavior under certain circumstances. Asused herein, the term “switchable behavior” or “switching behavior”refers to a change in properties with a change in circumstances, forexample, a change from one set of properties during the transport phaseand another set of properties inside the fracture. Switching behaviorcan be seen, for example, when a particle demonstrates hydrophilicproperties in the fracturing fluid and adhesive properties when in placewithin the fractures. Such behavior can be triggered by circumstanceslike the high closing pressures inside the fracture site so that theouter layer of the coating rearranges itself to exhibit moreadvantageous properties.

In more detail, the coated particle can switch from hydrophilic tohydrophobic when subjected to the high pressures inside the fractures.During the transport phase, when the hydrophilic covering of theparticle is exposed to the water-based fracturing fluid, it will tend tobe fully distended. As a result, the coating will provide the particlewith lubrication, facilitating its movement through the proppant slurry.When the particle has been conveyed to its destination within thefractures in the formation, the high pressures there will overcome thesteric repulsions of the external hydrophilic polymer chains, forcingthe outer layer to rearrange itself so that the inner layer is exposed.In embodiments, the switchable inner layer can be hydrophobic oradhesive, or both. As the inner layer becomes exposed, its propertiesmanifest themselves. If the inner layer has adhesive properties, forexample, it can fix the particles to each other to prevent theirflowback. This inner layer can also be configured to capture fines incase the proppant particle fails. Moreover, the residual intacthydrophilic groups present in the outer coating can allow easy flow ofoil through the proppant pack.

In embodiments, a coated proppant particle can be produced that bearsthe following layers of coating. First, a pressure-activated fixativepolymer can be used to coat the proppant substrate. This coating layercan be elastomeric, thereby providing strength to the proppant pack byhelping to agglomerate the proppant particles and distribute stress. Inaddition, this coating layer can encapsulate the substrate particles andretain any fines produced in the event of substrate failure. Second, ablock copolymer can be adsorbed or otherwise disposed onto the firstlayer of coating. The copolymer can have a section with high affinityfor the first polymeric layer, allowing strong interaction (hydrophobicinteraction), and can have another section that is hydrophilic, allowingfor easy transport of the proppant in the transport fluid.

In certain embodiments, a stronger interaction between the first andsecond coating layers may be useful. To accomplish this, aswelling-deswelling technique can be implemented. For example, the blockcopolymer can be adsorbed onto the surface of the elastomeric-coatedparticle. Then, the first coating layer can be swelled with small amountof an organic solvent that allow the hydrophobic block of the copolymerto penetrate deeper into the first coating layer and to become entangledin the elastomeric coating. By removing the organic solvent, the layeredpolymeric composite will deswell, resulting in a stronger interaction ofcopolymer with the elastomeric particle. A method forswelling-deswelling technique is set forth in “Swelling-Based Method forPreparing Stable, Functionalized Polymer Colloids,” A. Kim et al., J.Am. Chem. Soc (2005) 127: 1592-1593, the entire contents of which areincorporated by reference herein.

While the systems described herein refer to a two-layer coating system,it is understood that there may be multiple coating layers forming thecomposite proppant particles disclosed herein, with the each of themultiple coating layers possessing some or all of the attributes of thetwo coating layers described in the exemplary embodiments.

1. Particulate Substrate Materials

Composite proppant particles in accordance with these systems andmethods can be formed using a wide variety of proppant substrateparticles. Proppant particulate substrates for use in the presentinvention include graded sand, resin coated sand, bauxite, ceramicmaterials, glass materials, walnut hulls, polymeric materials, resinousmaterials, rubber materials, and the like. In embodiments, thesubstrates can include naturally occurring materials, for examplenutshells that have been chipped, ground, pulverized or crushed to asuitable size (e.g., walnut, pecan, coconut, almond, ivory nut, brazilnut, and the like), or for example seed shells or fruit pits that havebeen chipped, ground, pulverized or crushed to a suitable size (e.g.,plum, olive, peach, cherry, apricot, etc.), or for example chipped,ground, pulverized or crushed materials from other plants, such as corncobs. In embodiments, the substrates can be derived from wood orprocessed wood, including but not limited to woods such as oak, hickory,walnut, mahogany, poplar, and the like. In embodiments, aggregates canbe formed, using an inorganic material joined or bonded to an organicmaterial. Desirably, the proppant particulate substrates will becomprised of particles (whether individual substances or aggregates oftwo or more substances) having a size in the order of mesh size 4 to 100(US Standard Sieve numbers). As used herein, the term “particulate”includes all known shapes of materials without limitation, such asspherical materials, elongate materials, polygonal materials, fibrousmaterials, irregular materials, and any mixture thereof.

In embodiments, the particulate substrate can be formed as a compositefrom a binder and a filler material. Suitable filler materials caninclude inorganic materials such as solid glass, glass microspheres, flyash, silica, alumina, fumed carbon, carbon black, graphite, mica, boron,zirconia, talc, kaolin, titanium dioxide, calcium silicate, and thelike. In certain embodiments, the proppant particulate substrate can bereinforced to increase its resistance to the high pressure of theformation which could otherwise crush or deform them. Reinforcingmaterials can be selected from those materials that are able to addstructural strength to the proppant particulate substrate, for examplehigh strength particles such as ceramic, metal, glass, sand, and thelike, or any other materials capable of being combined with aparticulate substrate to provide it with additional strength.

In certain embodiments, the proppant particulate substrate can befabricated as an aggregate of two or more different materials providingdifferent properties. For example, a core particulate substrate of adense material, preferably having high compression strength, can becombined with a material having a lower density than thehigh-compression-strength material. In one embodiment, the materialhaving high compression strength is a dense material. A “densematerial”, as this term is used herein, is a material having a densitygreater than about 1.5 g/cm³, preferably in the range of 1.5 to 3 g/cm³.A “lower density material” is a material having a density less than thedensity of the dense material. In an embodiment, the lower densitymaterial has a density which is from about 0.1 to about 2.5 g/cm³ lessthan that of the dense material. In an embodiment, the lower densitymaterial has a density less than about 1.5 g/cm³. The combination ofthese two materials as an aggregate can provide a core particle havingan appropriate amount of strength, while having a lower density than thedense material. Preferably, the lower density material is buoyant in themedium in which the proppant is to be suspended. In one embodiment, themedium is a fracturing fluid. The fracturing fluid can be water or anaqueous solution having a density from about 1 g/cm³ to about 1.4 g/cm³.As a lower density particle, it can be suspended adequately in a lessviscous fracturing fluid, allowing the fracturing fluid to be pumpedmore easily, and allowing more dispersion of the proppants within theformation as they are propelled by the less viscous fluid into moredistal regions. High density materials used as proppant particulatesubstrates, such as sand, ceramics, bauxite, and the like, can becombined with lower density materials such as hollow glass particles,other hollow core particles, certain polymeric materials, andnaturally-occurring materials (nut shells, seed shells, fruit pits,woods, or other naturally occurring materials that have been chipped,ground, pulverized or crushed), yielding a less dense aggregate thatstill possesses adequate compression strength.

As used herein, the term “buoyant” refers to particles or materialshaving either neutral or positive buoyancy with respect to thesuspending medium, such that the particles have a lower density than thesuspending medium and they do not settle in the direction of gravity.

Aggregates suitable for use as proppant particulate substrates can beformed using techniques to attach the two components to each other. Asone preparation method, a proppant particulate substrate can be mixedwith a lower density material having a particle size similar to the sizeof the proppant particulate substrates. The two types of particles canthen be mixed together and bound by an adhesive, such as a wax, aphenol-formaldehyde novolac resin, etc., so that a population of doubletaggregate particles are formed, one subpopulation having a proppantparticulate substrate attached to another similar particle, onesubpopulation having a proppant particulate substrate attached to lowerdensity particle, and one subpopulation having a low density particleattached to another lower density particle. The three subpopulationscould be separated by their difference in density: the firstsubpopulation would sink in water, the second subpopulation would remainsuspended in the liquid, and the third subpopulation would float.

In other embodiments, a proppant particulate substrate can be engineeredso that it is less dense by covering the surface of the particulatesubstrate with a lower density coating comprising, for example, a foamymaterial. The thickness of the foamy material can be designed to yield acomposite that is effectively neutrally buoyant. To produce such acoated proppant particulate, a particle having a desirable compressionstrength can be coated with one reactant for a foaming reaction,followed by exposure to the other reactant. With the triggering of foamformation, a foam-coated proppant particulate will be produced.

As an example, a water-blown polyurethane foam can be used to provide acoating around the particles that would lower the overall particledensity. To make such a coated particle, the particle can be initiallycoated with Reactant A, for example a mixture of one or more polyolswith a suitable catalyst (e.g., an amine). This particle can then beexposed to Reactant B containing a diisocyanate. The final foam willform on the particle, for example when it is treated with steam whilebeing shaken; the agitation will prevent the particles fromagglomerating as the foam forms on their surfaces.

The foamy material can be applied to an uncoated proppant particle or aproppant particle having a single coating layer, such as an innercoating layer and optional intermediate layers as described herein. Inan embodiment, the foamy material is applied to a composite proppantparticle of the invention. In another embodiment, the proppant particleis a composite particle of the invention and the foamy layer ishydrophilic and is the outer layer of the composite particle.

2. Inner Polymeric Layer

In designing the polymers for the inner polymeric layer, a variety ofpressure-sensitive adhesive polymers can be used, having differentfunctionalities and molecular weights. As used herein, the inner polymerlayer is the first coating layer that is applied to the proppantparticulate substrate. Polymeric design for this inner polymeric layercan be directed by such variables as chemical resistance, ultimateadhesion, service temperature, and the like, so that a coating materialcan be selected that is targeted to the projected proppant usagetemperature. The coating can be optimized to produce strong adhesionamong proppant particles at different temperatures. For example, informulating the inner polymeric layer, it will be recognized that thetemperature in the formation is relatively high (from 30° to 100° C.),so that an adhesive would need to be designed to withstand such hightemperatures while still retaining its stickiness. A formulation may berequired comprising polymers with relatively high glass transitiontemperatures, for example, in order to withstand these hightemperatures.

In embodiments, coating thickness can be varied, which can have specificeffects on the strength of adhesion of the proppant particles as well.Appropriate coating methods can include solution coating or in-situcoating where the polymer is synthesized in the presence of the proppantsubstrate particle.

In embodiments, the inner polymeric layer can be made from a phenolicresin, an epoxy resin, a furan resin, a phenolic formaldehyde resin, amelamine formaldehyde resin, a urethane resin, a phenolic, furan resinmixture, a urea-aldehyde resin, a urethane resin, a furan/furfurylalcohol resin, a phenolic/latex resin, a polyester resin, an acrylateresin, or a combination of two or more thereof. In another embodiment ofthe invention, the inner polymeric layer with adhesive material can be athermoplastic resin. Examples of suitable thermoplastic resins include:styrene block copolymers such as: SBS (styrene-butadine-styrene), SIS(styrene-isoprene-styrene), SEBS (styrene-ethylene/butylene-styrene),SEP (styrene-ethylene/propylene); ABS copolymers (i.e.,acrylonitrile-butadiene-styrene); EVA (ethylene vinyl acetate)copolymers; acrylic polymers; vinyl ethers; and silicone rubbers. Acommercial thermoplastic, for example the ENABLE family of productsavailable from ExxonMobil Chemical Co, can be used; these materials area family of n-butyl acrylate copolymers (e.g., ENABLE EN 33900 and 60ENABLE 33330).

In embodiments, these materials can be mixed with other resins(tackifiers) that will increase their stickiness. Examples of tackifiersare: rosins and their derivatives, terpenes and their derivatives,shellac resin, small molecular weight aliphatic, cycloaliphatic andaromatic resins (less than 10 carbons), terpene-phenol resin, saturatedhydrocarbon resin. As an example of composition, the tackifier agent cancomprises 30-70% by weight of the combined weight of tackifier agent andthermoplastic resin.

In embodiments, the inner polymeric layer can be applied to theparticulate substrate by methods familiar to artisans of ordinary skill.For example, the application of the inner layer can be performed bysolution coating or by 100% solid coating (no solvent needed). Inembodiments, the inner layer can be applied in an amount of 0.25 to 10weight percent of the proppant substrate, for example, in an amount of0.5 to 5 percent.

3. Outer Coating Layer

It is desirable to impart hydrophilic features to the coated particle.In the aqueous environment of the hydraulic fracturing fluid, ahydrophilic coating can create a thin, water-like layer on the surfaceof the particle, making it slippery and reducing the friction betweenparticles. This can facilitate the transport of the particles in thefluid.

A second coating layer can be applied as an outside layer to provide thedesirable hydrophilic features to the overall particle. In otherembodiments, one or more intermediate layers can be applied to theparticle, then the outermost hydrophilic layer can be provided. In thedescription that follows, the second coating layer forms the outercoating of the particle. It is understood, though, that the outerhydrophilic coating may be applied to any number of inner, intermediatelayers, while still maintaining the advantageous properties of theparticles in accordance with the present disclosure.

In embodiments, the outer layer can be partially or wholly formed from apolymer. For example, a suitable block copolymer can be designed havinghydrophobic and hydrophilic sections or regions. Variables involved incopolymer design include the molecular weight of polymer, ratio ofhydrophobic to hydrophilic section, and the functionalities of thecopolymer. The outer layer, for example a second coating layer, can beadsorbed onto the first layer or onto an intermediate layer usingconventional methods of polymer adsorption, as would be known in theart, and/or swelling-deswelling using organic solvents.

In embodiments where a polymer layer is used as the outer coating layer,the polymer coating can be made from hydrophilic polymers like ethyleneoxide/propylene oxide (EO/PO) copolymers, polyvinyl acetate,polyethylene-co-vinyl acetate, polyvinyl alcohol, polysaccharides, andthe like. In embodiments, the outer layer can be fabricated from blockcopolymers having hydrophilic and hydrophobic segments. Such materialscan be diblock, triblock or multiblock copolymers. For example, anethylene oxide/propylene oxide block copolymer can be used, for examplethe Pluronic family of copolymers (BASF). As another example, Guerbetalcohol ethoxylates, lauryl alcohol-tridecyl alcohol-stearylalcohol-nonylphenol- or octylphenol-ethoxylates, for example theLutensol family of products (BASF). In embodiments, the selectedmaterials will have a high hydrophilic-lipophilic balance, so that theproduct is substantially more hydrophilic than hydrophobic. Examples ofsuch materials include certain stearyl alcohol andnonylphenol-ethoxylates.

In embodiments, the outer layer can be applied to the first layer or toan intermediate layer using a swelling-based method. According to such amethod, the first layer or an intermediate layer can be exposed to asolvent that can swell this layer without dissolving it. The polymer forthe second layer, having both hydrophilic and hydrophobic segments, canbe dissolved in the same solvent. When the solution bearing the secondlayer polymer is put into contact with the particles bearing the swollenfirst layer, the hydrophobic segments of the polymer will tend tointeract with the hydrophobic first layer, resulting in entanglement ofthe two hydrophobic entities. When the solvent is removed, the firstlayer will deswell, locking the hydrophobic attachments of thesecond-layer polymer in place. The hydrophilic segments of thesecond-layer polymer will be directed outwardly, away from the innerlayer.

In other embodiments, the outer layer can be formed by chemical reactionor modification of the inner polymer layer or an intermediate layer. Forexample, the outer surface of the inner polymer layer or an intermediatelayer can be oxidized, etched, epoxidized, ethoxylated, hydrolyzed, orotherwise coated to protect the inner polymer layer from the fluidenvironment in the fracture.

EXAMPLES Example 1 Preparation of Inner Polymeric Layer Material

A material suitable for the inner polymeric layer of a coated proppantparticle can include a pressure sensitive adhesive. Such an adhesive canbe prepared as follows:

15 gm of a 10 wt % tetrahydrofuran solution ofPolystyrene-block-polyisoprene-block-polystyrene (22 wt % styrene and aviscosity of 750-1250 cps at a 25% solution in toluene at 25° C.)(Aldrich) was mixed with 15 g of a 10 wt % tetrahydrofuran solution ofColophony rosin gum (acid value of 150-170) (Aldrich).

Example 2 Coating Sand Particles with Inner Polymeric Layer Material

Quartz sand particles (50-70 mesh particle size) (Aldrich) were coatedwith 0.5 wt % of the material prepared in Example 1 by solution coatingusing the following procedure:

30 gm of the solution from Example 1 was added to approximately 270 gmof tetrahydrofuran in a 1 liter round bottom flask, and mixedthoroughly. Once the solution was homogeneous, 300 g of the quartz sandwas added. The solvent was then evaporated under vacuum in a rotaryevaporator to yield sand particles with adhesive properties. The coatedsand particles were further characterized in Examples 4 and 5 below.

Example 3 Applying an Outer Polymeric Layer to the Coated Sand Particles

To 100 ml of ethyl alcohol (Aldrich) was added 0.5 g of PLURONIC® F127Prill (BASF 100 Campus Drive, Florham Park, N.J. 07932). The mixture wasstirred under mild heat until the entire solid dissolved. To theresulting solution was added 100 g of the product from Example 2. Themixture was stirred for 30 minutes and the solvent evaporated undervacuum in a rotary evaporator to yield sand particles with adhesive andhydrophilic properties. The coated sand particles were furthercharacterized in Examples 4 and 5 below.

Example 4 Characterization of Frictional Forces between Coated SandParticles

Coated sand particles prepared in accordance with Examples 2 and 3 wereanalyzed by the high solids rheology test. Control samples (Aldrich)without any treatment (as-received sand) were tested along with thesamples prepared in accordance with Example 2 and Example 3. All sampleswere analyzed in the dry and water wet state (a minimum amount of waterwas added to the dry sample to wet all the particles). A Brookfieldmodel DV-III Viscometer with a #4 spindle was employed. The test wasperformed by rotating the spindle at 20 rpm for 1 minute: then theapplied torque was turned off and the maximum torque at this point wasrecorded. The results of these tests are set forth in Table 1.

TABLE 1 Sample % Torque - Maximum As-received sand - Dry   28.1As-received sand - Wet   42.8 Example 2 - Dry >110⁽*⁾ Example 2 - Wet>110⁽*⁾ Example 3 - Dry >110⁽*⁾ Example 3 - Wet 50 ⁽*⁾110% Torque is themaximum reading for the instrument

The maximum torque, as measured by these tests, gives an indication ofthe frictional forces between particles. The higher values of Maximum %Torque for particles in Example 2 and 3 compared to the as-receivedsand, indicate that the coating applied to the sand act as an adhesive,consolidating the particles together and increasing the maximum torqueneeded to rotate the spindle. The comparison of the Maximum % Torque ofthe wet samples in Example 2 with the samples prepared in Example 3shows a smaller value for Example 3 sample, indicating that the secondlayer (which is hydrophilic) has affected the surface of the particlesso as to provide better lubrication when water-wet.

Example 5 Characterization of Cohesiveness of Coated Sand Particles

A 5 gm sample of the particles prepared in each of Examples 2 and 3, and5 gms of the control samples (Aldrich) as received was placed on astainless steel circular plate (2.24 inches diameter). Another plate ofsame shape and size was placed on top of the sample. Then the 2 platescontaining the sample were placed in a Carver Laboratory Press (Model C)and the desired pressure applied (1269, 2539 or 3808 psi) for 1 minute.Next, the top plate was carefully removed and the amount of sampleadhered to it weighted. The bottom part of the mold was then elevated atan angle of approximately 45° and tapped several times. The amount ofsample adhering to the plate was then removed and weighed. The resultsare set forth in Table 2.

TABLE 2 Example 2 Example 3 As-received sand Weight/g Weight/g Weight/gWeight/g Weight/g Weight/g Pressure/psi Top plate Bottom plate Top plateBottom plate Top plate Bottom plate 1269 0 0.561 0 0.522 0 0 2539 02.335 0 0.581 0 0 3808 0.411 3.711 0 1.009 0 0

The results in Table 2 indicate that the as-received sand does not stickto the plates at all. For samples from Example 2 and 3 there is afraction of the sample that remained on the plates, demonstrating thecohesiveness of the particles. At the higher applied pressure, a largeramount of the samples remained on the plates. More of the samples fromExample 2 remained attached to the plates at both pressure levels,indicating higher cohesiveness than for Example 3.

Example 6 Preparation of Polyurethane-Foam-Encapsulated Sand Particles

Polyurethane-foam-encapsulated sand particles can be prepared asfollows:

A prepolymer of poly(propylene oxide) glycol and 2,4-toluenediisocyanatecan be prepared by placing 200 g of dried polypropylene glycol(molecular weight 2,000, hydroxyl number 56.1) in a reactor equippedwith a stirrer, condenser with drying tube, thermometer and gas inletwith nitrogen flush. Next, 0.8 g of water can be added. After stirringthe mixture for a few minutes, 29.4 g of toluene diisocyanate can beadded. The mixing can be continued and the temperature increased to 110°C. for 1 hour. The resulting material is a prepolymer ready to be coatedand foamed onto the sand particles.

25 gm of 50-70 mesh sand can then be placed in a plastic container. Tothis can be added 1 g of the prepolymer prepared in the previous stepand 1 drop of triethylamine. The mixture can be mixed in a speed mixerat 3,000 rpm for 1 minute to yield homogeneously coated sand particles.Next the coated particles were placed in an oven at 100° C. for 30minutes while steam is flown though it in order to finalize the foamingprocess. The foam as described in this Example would be designed to havea density of approximately 40 kg/m³. The coating encapsulating the sandparticles would comprise about 50% by volume of the total volume of thecoated particle. The thickness of the coating has been designed to givea neutral buoyant particle according to Stokes' law.

Example 7 Preparation of Sand Particle Attached to Buoyant Particles

This experiment shows a method of attaching low density particles tosand to yield composite particles with improved buoyant properties. In aplastic container was placed 2.68 gm of sand (50-70 mesh size fromAldrich), 0.63 g of 3M™ Glass Bubbles K20 (hollow glass microsphereswith density 0.2 g/cm³), and 0.3 g of a paraffin wax (53-57° C.) fromAldrich. The mixture was mixed in a speed mixer at 3,000 rpm for 5minutes. The resulting product was a homogeneous free flowing solid. Thesample was characterized by comparing the settling rate of theglass-bubbles-treated sand with the as-received sand in water (control).To characterize the samples, 2 burettes were filled with water; andapproximately 0.5 g of the treated and as-received sand was added toeach burette. The settling rate for each sample was monitored byfollowing the time needed for the particles to reach the bottom of theburette. The results indicated that for the treated sand the settlingrate was approximately half of the as-received sand, showing theimproved buoyant properties of the treated sand.

EQUIVALENTS

While specific embodiments of the subject invention have been disclosedherein, the above specification is illustrative and not restrictive.While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims. Many variations of the inventionwill become apparent to those of skilled art upon review of thisspecification. Unless otherwise indicated, all numbers expressingreaction conditions, quantities of ingredients, and so forth, as used inthis specification and the claims are to be understood as being modifiedin all instances by the term “about.” Accordingly, unless indicated tothe contrary, the numerical parameters set forth herein areapproximations that can vary depending upon the desired propertiessought to be obtained by the present invention.

What is claimed:
 1. A treatment fluid comprising an aqueous fracturingfluid and composite proppant particles, wherein the composite proppantparticles are suspended in the fracturing fluid and wherein thesuspended composite proppant particles comprise: a proppant particulatesubstrate, an inner polymeric coating deposited on the particulatesubstrate wherein the inner polymeric coating comprises a pressuresensitive adhesive polymer, and an outer polymeric coating depositedupon the inner polymeric coating or on an optional intermediate coatingwherein the outer polymeric coating comprises a hydrophilic polymerselected from the group consisting of EO/PO copolymers, polyvinylacetate, polyethylene-co-vinyl acetate, polyvinyl alcohol,polysaccharides and combinations thereof, wherein this outer polymercoating is selected so that (a) when this outer polymer coating isexposed to a water-based fracturing fluid, it distends and impartshydrophilic properties to the coated particles, and (b) when thecomposite proppant reaches its destination within the fracture, thisouter polymer coating rearranges itself to expose the inner polymericlayer, thereby enabling multiple composite proppant particles to fixthemselves to one another.
 2. The treatment fluid of claim 1, whereinthe proppant particulate substrate comprises sand.
 3. The treatmentfluid of claim 1, wherein the proppant particulate substrate comprisesceramic.
 4. The treatment fluid of claim 1, wherein the proppantparticulate substrate has a specific gravity from about 1.0 to about 2.7g/cc.
 5. The treatment fluid of claim 1, wherein the inner polymericcoating comprises a polymer selected from the group consisting ofstyrene block copolymers including SBS (styrene-butadiene-styrene), SIS(styrene-soprene-styrene), SEBS (styrene-ethylene/butylene-styrene) andSEP (styrene-ethylene/propylene); ABS copolymers includingacrylonitrile-butadiene-styrene); EVA (ethylene vinyl acetate)copolymers; acrylic polymers; vinyl ethers; and silicone rubbers.
 6. Thetreatment fluid of claim 1, wherein the outer polymeric coating isselected so that when this outer polymer coating is exposed to awater-based fracturing fluid, it distends to exhibit hydrophilicproperties capable of facilitating movement of the composite proppantwhen wet through the subterranean geological formation.
 7. A treatmentfluid comprising an aqueous fracturing fluid and composite proppantparticles, wherein the composite proppant particles are suspended in thefracturing fluid and wherein the suspended composite proppant particlescomprise: a proppant particulate substrate, an inner polymeric coatingdeposited on the particulate substrate wherein the inner polymericcoating comprises an adhesive polymer, and an outer polymeric coatingdeposited upon the inner polymeric coating or on an optionalintermediate coating, wherein the outer polymeric coating comprises ahydrophilic polymer selected from the group consisting of EO/POcopolymers, polyvinyl acetate, polyethylene-co-vinyl acetate, polyvinylalcohol, polysaccharides and combinations thereof, wherein this outerpolymer coating is selected so that (a) when this outer polymer coatingis exposed to a water-based fracturing fluid, it distends and impartshydrophilic properties to the coated particles, and (b) when thecomposite proppant reaches its destination within the fracture, thisouter polymer coating rearranges itself to expose the inner polymericlayer, thereby enabling multiple composite proppant particles to fixthemselves to one another.